All human endeavor has become dependent on the steady economical supply of petroleum products. While much effort has been expended at finding alternative energy sources, there are currently no known replacements for naturally occurring oil and natural gas In order to maintain the supply balance, it is essential that the petroleum industry replace the recovered oil and natural gas with proven reserves. Because oil is a finite resource, each discovery makes the next one more difficult. Not only are hydrocarbon reserves scarce and harder to find, they tend to be located in environments that are remote and hostile. These factors, scarcity and exploration difficulty, make it vastly more expensive to locate successful commercial quantities of hydrocarbons.
These circumstances make it absolutely essential to predict more accurately the subterranean geological structures that are likely to hold commercial quantities of hydrocarbons. The industry has come to rely on seismic prospecting, a technique used to record and develop as detailed a picture as possible of subterranean geology, in an effort to locate new sources of hydrocarbons. Seismic prospecting or seismic exploration produces a geological model describing the location and physical properties of the various earth layers. The accuracy of this model is strongly dependent on the accuracy of the data used to produce the model.
The seismic data used in preparing the model are collected using a system comprised of three main components: an input source, an array of detectors and a recording apparatus. The input source generates a pulse of acoustical energy which is intended to meet certain predetermined requirements of total energy, duration, frequency, amplitude and phase. The reflected pulse is then detected by an array of either geophones or hydrophones. The detector array converts the detected pulse to an electronic digital signal which is stored on a recording instrument. The recorded digital signals are then processed by computer to construct the geological model.
Because the seismic source directly influences the quality of the recorded seismic data, it becomes highly desirable to determine the signal characteristics of the source wavelet. Various techniques are known for assessing the source signature. One technique calls for the direct measurement of the wavelet produced by the source. While it may be possible to measure correctly the wavelet of a single source, it is far more difficult, if not impossible, to measure the wavelet for a source array (such as is most often used in practice). Techniques have therefore been developed that attempt to estimate the source wavelet. One technique attempts to record the marine source signature only in very deep water so that reflections are not included. This technique is costly in practice and of no advantage for land exploration.
Another technique suggested by Ziolkowski, et al., 47 Geophysics, 1413 (1982) calls for recording the pressure field near each gun in the array and then computing the far field wavelet signature from this information. One disadvantage of this technique is that the computation requires very accurate knowledge of the elements of the source array and sea surface reflection coefficient. Another disadvantage is that the hydrophones must be placed in very close proximity to the source which causes premature hydrophone failure. Also fluctuations in the source depth and changes in sea state may produce undetected changes in the array signatures as may also variations in the array geometry.
Another technique for estimating the acoustic wavelet is suggested by Loewenthal, et al., 33 Geophysical Prospecting, 956 (1985), but assumes that (1) the medium everywhere is known and (2) the medium was only a one-dimensional variation. This is not easily achieved in practice.
Another technique suggested by Sonnland in a paper entitled "2-D Deghosting Using Vertical Receiver Arrays" presented to the Society of Exploration Geophysics in 1986 suggests using a one dimensional acoustic wave equation with two independent vertical measurements of pressure to perform a combined designatured reverberation.
Another technique suggested by Hargreaves in a paper entitled "Far-Field Signatures by Wave Field Extrapolation" presented at the 46th Annual EAEG meeting in London, England in June, of 1984, presents a wave field extrapolation method for source signature identification.
One problem with the foregoing techniques is that only the wavelet radiated vertically downward is obtained while the complete radiation pattern of the source array is not determined. Another problem is that the wavelet estimation is contaminated by extrapolation artifacts due to the presence of the scattered wave field. The severity of these problems is such that this method is inadequate for advanced seismic processing techniques such as Amplitude vs. Offsets (AVO) studies in migration/inversion, both of which require accurate knowledge of the wavelet and its radiation pattern.